Paracanthopterygian

Paracanthopterygian (superorder Paracanthopterygii), any member of a large group of predatory, primarily marine fishes that forms one of about six major branches of the Teleostei, or bony fishes. Approximately 1,340 living species of paracanthopterygian fishes have been described. They range in length from just a few centimetres to roughly 2 metres (about 7 feet). Well-known forms include the anglerfish (order Lophiiformes) and the cod (order Gadiformes).

General features

In general body form there is considerable diversity, but ichthyologists have classed the Paracanthopterygii as a discrete group largely on the basis of a distinctive musculature of the jaws, the structure of the caudal (that is, at the tail end) vertebrae, and the placement of the pelvic fins. (In this group the pelvic fins are usually located in the midbody region or even farther toward the head.)

Body plans of representative members of the orders Gobiesociformes, Lophiiformes, Percopsiformes, …

Body plans of representative members of the orders Batrachoidiformes (toadfish) and Gadiformes.

Encyclopædia Britannica, Inc.

Most of the orders are primarily marine, with worldwide distribution; the percopsiforms, however, occur only in the freshwater environments of North America. Batrachoidiforms occur mainly in tropical and temperate shallow water along continental coasts and to a limited extent in fresh water. Gadiforms are represented by both shallow-water and deep-sea types. The most widely known gadiforms are the commercially important species and the only economically important paracanthopterygians: the true cods (Gadus), hakes (Merluccius, Urophycis), haddocks (Melanogrammus), pollocks (Pollachius), and whitings (Merlangius). All are abundant in waters of the continental shelf of the North Atlantic, where they have been commercially fished for centuries from both Europe and North America. Lophiiforms live in shallow waters of tropical reefs as well as in the ocean depths.

The largest of the Paracanthopterygii are the codfishes, which grow to about 2 metres (about 7 feet) in length and attain weights that may exceed 90 kg (about 200 pounds). Certain goosefishes (Lophiiformes) reach a length of about 2 metres and a body weight of 35 kg (about 75 pounds); other lophiiforms are as small as 2.5 cm (about 1 inch) long. Batrachoidiforms grow to about 30 cm (about 12 inches) in length, and the largest percopsiforms are about 15 cm (6 inches) long.

Natural history

Life cycle and reproduction

Eggs of the oyster toadfish (Opsanus tau) of the western Atlantic—one of the most carefully studied batrachoidiforms—are laid in dark recesses of all sorts, including sunken tin cans and shoes. The male guards the eggs and young for about three weeks, after which the young begin life on their own. This fish gets its name from the fact that some have been found living in live oysters. Luminous organs known as photophores, numbering several hundred and set in long horizontal rows, are believed to be sexual attractants in the midshipman (Porichthys)—so named because the organs resemble rows of bright buttons on a naval uniform. The northern midshipman (P. notatus), a common species on the eastern Pacific coast, spawns in shallow water, attaching its eggs to a rocky surface. The male guards the eggs. Like other batrachoidiforms, the midshipman lives and grows on the ocean bottom.

Most species of codfishes (which comprise some 70 species of Gadiformes) migrate over long distances. They gather in late winter and early spring to spawn, each species going to a particular area. The periodic movements are closely related to seasonal variations in water temperature. The fecundity of some codfish species is prodigious. The European ling (Molva molva) may deposit as many as 60 million eggs each season. The eggs and larvae of most species are found among the plankton (small aquatic organisms that drift with ocean currents). Weeks or months elapse before the eggs hatch. Young codfishes are commonly found in very shallow water, but they move into deeper water as they become older. The eggs of grenadiers (family Macrouridae), a bottom-feeding group of cods, are believed to be laid near the ocean bottom; the buoyant eggs rise partway to the surface. The larvae are known mainly from below 100 fathoms (about 180 metres, or 600 feet); older larvae occur at greater depths.

In the Mediterranean pearlfish (Carapus acus), a member of the order Ophidiiformes (family Carapidae), clumps of eggs released by the female in late summer appear at the surface and hatch into a specialized larva, the vexillifer, which lives amid the plankton. After attaining a length of about 7 to 8 cm (about 3 inches), the vexillifer transforms to another larval stage, the tenuis, descends to the bottom, and becomes a parasite in a sea cucumber (Holothuria tubulosa or Stichopus regalis). The tenuis, apparently dependent upon its host for survival, undergoes a further transformation to the juvenile stage; in the process its length decreases from 20 to 10 cm (8 to 4 inches). The Mediterranean pearlfish is believed to pass most of its life in the host. Very little is known of the general biology of reproductive habits of the brotulas and cusk eels (family Ophidiidae). Ophidiids are both oviparous (egg-laying) and viviparous (live-bearing). The males of some viviparous species produce spermatophores (sperm cases).

The lophiiforms are primarily bottom fishes as adults, but many produce floating rafts of eggs. The eggs of the deep-sea anglerfishes (suborder Ceratioidei) have not been studied or observed, but it is believed that they float to the surface; the larvae occur in surface waters, gradually descending to deeper waters as they grow older. The females of the deep-sea anglers are 3 to 13 times as large as the males. Females have an illicium, or “fishing pole,” which is a modified spine of the dorsal, or back, fin that has moved forward onto the top of the head. At the tip of the illicium is a fleshy enlargement, the esca, used to lure prey within range of capture. (The illicium and esca are generally also present in male anglerfishes but do not appear in members of suborder Ceratioidei.) The esca is commonly luminous; the female also has other light-producing organs. In 1922 a specimen of the anglerfish Ceratias holboelli was discovered with small specimens attached to its abdomen that were thought to be its young. A few years later, similar finds led to the discovery that the smaller fish were really mature males living parasitically on the female. Further investigation showed that the males, soon after their transformation from the larval state, bite an older, larger female, after which the female and male tissues unite; the separate circulatory systems join; and the male becomes a permanent appendage of the female.

Little is known of the reproductive habits of the percopsiforms. They are known to spawn in the spring of the year in shallow water.

Ecology and behaviour

All batrachoidiforms are bottom dwellers. True toadfishes (Batrachoidinae, about 52 species) occur in shallow or moderate depths along continental coasts; some ascend rivers. The oyster toadfish lives under rocks or amid debris, awaiting prey of almost any type, which is taken with a sudden snap. Venomous toadfishes (Thalassophryninae, about 11 species) are restricted to the coasts and rivers of Central and South America. Because of their sluggish habits, these fishes are sometimes stepped on by people and can inflict painful wounds. Midshipmen (Porichthyinae, about 15 species), which are restricted to tropical and temperate coasts of the Americas, are unusual in being shallow-water fishes with photophores, a feature generally found in deepwater forms. Most midshipmen occur at depths of less than 50 fathoms (1 fathom equals 6 feet), and all are found in water shallower than 200 fathoms.

Gadiform fishes of the family Gadidae (about 70 species) are all marine species, except for the burbot (Lota lota); some, however, ascend rivers with the tides. Bottom dwellers, they occur on the continental shelves from shallow water to about 200 fathoms and, although distributed throughout the oceans, are most numerous in the eastern North Atlantic. Deep-sea cods (Moridae, about 105 species) are cold-water bottom fishes, living at greater depths along the continental slopes. Grenadiers (Macrouridae, about 350 species), typically bottom fishes, live along the continental slopes at depths of 100 to 1,000 fathoms. Few species are cosmopolitan in distribution, but the group as a whole is widely distributed in tropical and temperate latitudes. Four species of eel cods (Muraenolepididae) are confined to Antarctic seas, and, like the cods, they are bottom fishes, living at moderate depths.

In ophidiiforms, the pearlfishes (Carapidae, about 31 species) are marine, mainly tropical, shallow-water eel-like fishes adapted to living inside the body of various invertebrates. They have been collected from a variety of hosts, including tunicates, oysters, and sea cucumbers. Brotulas and cusk eels (Ophidiidae, about 250 species) are mainly bottom dwellers. Some are shallow-water species with nocturnal habits, but the group as a whole is the dominant family of teleosts at depths greater than 2,000 fathoms. Some have been taken at depths of about 4,000 fathoms, the greatest depth at which any form of fish life is known.

Of the lophiiforms, the ceratioids, or deep-sea anglerfishes, are the only abyssal (deep-sea) forms. They occur primarily at depths of 1,000 to 3,000 fathoms. Unlike other lophiiforms, they are midwater fishes, are uniformly black, and have no pelvic fins. They are apparently feeble swimmers, depending primarily on their light organs to attract prey. Some (Melanocetus, Linophryne) are known to swallow fishes several times their own length, accommodating them in a highly distensible stomach. Crustaceans and other invertebrates are also eaten. In addition, the so-called frogfishes (Antennariidae, about 42 species) are shallow-water forms that commonly inhabit coral reefs. Frogfishes typically have highly varied colour patterns, and some species are able to change colours. In habit they are sedentary but can use their fins to walk on the bottom and to climb over obstacles. The tropical sargassum fish (Histrio histrio), so called because it lives amid floating brown algae of the genus Sargassum, clings to the branches of algae with prehensile pectoral fins as it searches for prey, which is sucked into the mouth by powerful jaws and expandable cheeks.

The lophiiform group known as goosefishes (Lophiidae, about 25 species) seldom occurs in shallow water, preferring instead the moderate depths (10 to 500 fathoms) along the continental slopes of tropical and temperate regions. The batfishes (Ogcocephalidae, about 60 species) are mainly deepwater lophiiforms, but some (Ogcocephalus, Halieutichthys) are regularly found in water only a few feet deep. Like frogfishes, they walk on the bottom using their pectoral fins. Batfishes are awkward swimmers and, when disturbed, tend to bury themselves in the bottom rather than swim away.

Percopsiforms (about nine species) may live under conditions of dim light. Cave fishes, with eyes reduced to nonfunctional rudiments, have elaborate systems of sense organs in the skin of the head, body, and tail; they live in total darkness. Because of the secretive habits of percopsiforms, little is known about species other than the trout-perch (Percopsis omiscomaycus), which is widely distributed in central North America and is abundant in some of the Great Lakes, where it occurs in clear water to a depth of about 35 fathoms.

Form and function

Batrachoidiforms generally have two dorsal fins: a small anterior fin, usually with two spines, and a long posterior fin. In venomous species the hollow fin spines form an efficient apparatus for the injection of venom. A similar spine is found on each cheek (operculum).

Among the gadiforms, the dorsal and anal fins of some deep-sea cods are distinctively arranged as three dorsals and two anals. This arrangement also occurs in some codfishes (Gadidae). The macrourids are characterized by a long, tapering tail. A tubular light organ containing luminescent bacteria is sometimes present along the ventral midline of both sexes. All but a few species of macrourids have a well-developed swim bladder; in the males of some species and in some codfishes the swim bladder is equipped with drumming muscles, indicating that sound can be produced. In the bregmacerotids and muraenolepidids there are two dorsal fins, with the anterior fin represented by a single ray. Brotulas may have the pelvic fins either present or absent, but cusk eels have them anterior in position, under the lower jaw. The pelvic fins of both types, kept in continuous probing motion as the fish swims just off the bottom, aid in detecting food.

Some lophiiforms are unique among teleostean fishes in having only two gills. The ogcocephalids are somewhat flattened anglers, in this respect resembling lophiids rather than the balloonlike antennariids; they are distinctive in having the illicium, when not in use, concealed in a tube (illicial cavity) between the eyes and over the mouth. Like most anglerfishes, they lack typical scales but are distinctively equipped with bony tubercles (projections) and spines imbedded in their skin.

Evolution and paleontology

Fossil batrachoidiforms include only material from lower Pliocene marine deposits (about 3.6 million to 5.3 million years old) of North Africa. These fossils are similar to a living species, Batrachoides didactylus.

Fossil gadiforms are relatively numerous and are known primarily from Northern Hemisphere marine deposits from the Paleogene and Neogene between 65.5 million and 2.6 million years ago. A Paleocene fossil between 56 million to 65.5 million years old was identified as a codlike fish. Fossils linked to the families Bregmacerotidae, Gadidae, and Macrouridae, along with the Ophidiidae (order Ophidiiformes), date back to Oligocene and Miocene times (34 million to 5.3 million years ago); some fossil bregmacerotids and gadids have even been traced as far back as the Eocene Epoch (56 million to 34 million years ago). Furthermore, many fossil ear stones (otoliths) and scales, beginning with specimens from the Cretaceous (146 million to 65.5 million years ago), are similar to those of gadiforms. Fossil lophiiforms include two species from Eocene marine deposits of Europe and one species from Pliocene marine deposits of North Africa. One of these species from the Eocene has been identified as a goosefish (Lophiidae) and the other a frogfish (Antennariidae).

Fossil percopsiforms include three genera from Cenozoic freshwater deposits of North America and one genus (Sphenocephalus) from Cretaceous marine deposits of Europe. Of the North American genera, two (Amphiplaga and Erismatopterus) that date from the middle Eocene have been identified as trout-perches (Percopsidae). The third genus (Tricophanes), which dates to near the Oligocene-Miocene boundary, has been established as a pirate perch (Aphredoderidae). The relationships between Sphenocephalus and other genera are obscure.

Reduced caudal skeleton; elongate body; altered head and jaw structure. Very reduced fin spines. Marine, worldwide. About 10 families, with about 75 genera and 555 species. Length 7 to about 200 cm (2.75 inches to about 6.5 feet). Paleocene to present. Early gadiforms were similar in structure to early percopsiforms, but almost all remained marine and subsequently specialized into a variety of environments.

Critical appraisal

The interrelationships of the groups listed here as paracanthopterygians are not yet well established, and the classification given here is provisional. There is considerable agreement that trout-perches (Percopsiformes) and cods (suborder Gadoidei) are closely related, and this agreement may be considered the basis of the Paracanthopterygii. What other fishes should be included in the superorder is a question receiving continued study. The clingfishes (Gobiesociformes) and eelpouts (Zoarcidae) were once classified as paracanthopterygians, but they are now considered to be perciform fishes. Similarly, the order Polymixiiformes was once placed in the superorder Paracanthopterygii, but it is now considered to be the closest living relative of the Acanthomorpha, a clade that includes the Paracanthopterygii and its sister group, the Acanthopterygii. Debate also continues on whether the paracanthopterygian fishes are either close relatives of the atherinomorph fishes or more primitive members of the acanthomorphs.